The present invention relates in general to a technique for separating or recycling plastics, and in particular, to a new and useful method and apparatus for dealing with streams of physically commingled plastics, such plastic streams arising in any of a variety of reclaiming or recycling operations.
For the purpose of this disclosure, commingled plastics mean a mixture of chemically different plastics, wherein particles in the mixture consist primarily of single types of plastics, as might result when a variety of plastic packaging materials are mixed together and coarsely ground. Although not confined to such streams, an important application of the invention is to the commingled, post-consumer plastics that result when plastics are separated from ordinary household trash. Such plastic mixtures may also include minor amounts of paper, metals, glass and other substances. The mixture may also contain particles consisting of more than one kind of plastic, as for example when multiwall films or bottles are the source of the plastic particles. Such composite materials, including impact-modified plastics, may also be treated by the present invention.
Individual plastics have long been reclaimed as part of pre-consumer manufacturing operations. Commingled plastics, in particular those in household waste, have more commonly been land-filled or burned. The typical household waste is substantially composed of the following plastics: polyvinyl chloride (PVC), polystyrene (PS), low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP) and polyethylene terephthalate (PET). Some technology, primarily of European origin, extrudes commingled plastics into low-value articles, such as wood substitutes. The various techniques for directly utilizing commingled plastics reclaim at best a small portion of the value of the original component plastics. Typically, the value as a fuel or as a wood-substitute will be less than ten percent of the value of the original plastics. Incentive thus exists to separate a commingled plastics stream into its component polymers.
Commercial and semi-commercial recycling schemes have typically resorted to manual sorting of containers prior to a grinding step and further separation techniques such as flotation and hydrocycloning. This has been successfully applied to certain well-defined recycle streams such as returned, soft-drink bottles. Problems remain, however, even when these streams are separated from metals, paper, and other wastes. The major difficultly is that the plastics themselves are mixtures. This problem is of an increasing complexity with the advent of multilayered containers. Separation techniques have been even less successful for the complex mixtures typical of general post-consumer trash, and are unsuccessful whenever the component polymers have similar specific gravities.
Simple coextrusions of commingled plastics produce composites with little strength. A major reason for this is thermodynamic incompatibilities between the component polymers. Most pairs of polymers are incompatible and form two phases upon mechanical mixing. This incompatibility typically leads to poor physical properties unless one of the components is microdispersed in the other (see U.S. Pat. Nos. 4,594,371 and 4,666,961 to Nauman). To this end incentives exist to separate waste plastic streams into pure polymers or at least render innocuous certain components that may cause difficulties in fabrication or lead to poor physical properties in a mechanical blend.
An article which discloses work that focuses on the impact modification of polymers via flash evaporation and compositional quenching is Nauman, E. B., Ariyapadi, M., Balsara, N. P., Grocela, T., Furno, J., Lui, S., and Mallikarjun, R., "Compositional Quenching: A Process for Forming Polymer-in-Polymer Microdispersions and Interpenetrating Networks", Chem. Eng. Commun., 66, 29-55 (1988).
U.S. Pat. No. 4,031,039 to Mizumoto, et al. discloses a method for treating waste high-polymer mixtures where polystyrene (PS) is dissolved in cold xylene then dissolved mixed polyolefins are dissolved in hot xylene. A separate polar solvent is subsequently used to dissolve PVC. This constitutes a two-way separation with one solvent and a three-way separation with two solvents. Thus, Mizumoto, et al. does not disclose the possibility of dissolving three polymers at separate temperatures using a single solvent.
A need remains for a comprehensive scheme for separating a wide variety of commercial synthetic materials in an economic fashion to solve the ever increasing solid waste disposal problem.